Field of the Invention
The present invention relates to a method of determining a radius of protection associated with a respective navigation parameter of a hybrid inertial navigation system, and associated system.
Description of the Related Art
The present invention may be used in the aeronautical field, as in the remainder of the description, but may also be used in any other navigation field.
There is a problem in supplying safe navigation parameters (such as speed and attitude) to the pilot, i.e., in knowing the navigation parameter or parameters and obtaining a respective radius of protection associated with that parameter. A radius of protection gives an uncertainty associated with the navigation parameter: in other words, the pilot knows within a given probability that the error in respect of the navigation parameter cannot be greater than the radius of protection.
There is known the document US2014074397, which discloses a method of providing the integrity of a hybrid navigation system using a Kalman filter. The method consists in determining a main navigation solution for at least one of the roll, pitch, platform heading or vehicle heading parameters using signals from a plurality of GNSS (Global Navigation Satellite System) satellites and inertial measurements. Solution separation is used to determine a plurality of subsolutions for the main navigation solution. This method also includes the determination of a separation between the main navigation solution and each of the subsolutions and a discriminator for each of the separations. The method also includes the determination of a separation variance between the main navigation solution and each of the subsolutions, a satellite failure detection threshold based on the separation of the variances, and a limit of protection that delimits an error in the main navigation solution as a function of the threshold.
Such a method is shown in FIG. 1, in which the method receives as input measurements 1 of pseudo-distances PD1, PD2, . . . PDN supplied by a GNSS (Global Navigation Satellite System) and from the hybrid inertial navigation system inertial measurements 2 including position, speed, attitude and altitude. The method includes a main Kalman filter step FKP 3 and a plurality of secondary filter steps FKSi 4 (with i varying from 1 to n), and a step 5 of radius of protection computation and monitoring. The method supplies as output a safe position and an associated radius of protection 6 together with other safe navigation parameters 7.
Providing a safe speed or safe attitudes necessitates a model of the short-term variation of the input pseudo-distance measurements and an undetected satellite failure rate associated with these short-term variations has the following disadvantages.
The undetected satellite failure rate of 10−4/h is supplied in relation to amplitude errors and not short-term variations that can affect the speed or the attitudes.
In the standards there is no valid model of short-term variations of the pseudo-distances.
Such methods are subject to unmodelled short-term variations of GNSS errors, for example GPS errors (the latter are of course smoothed by the inertia but it is not possible to say in what proportion). The radius of protection associated with the speed covers only certain types of faults and not faults linked to short-term variations.
An object of the present invention is to alleviate these drawbacks, notably by making no hypothesis as to the short-term variations of the GPS measurements.
One aspect of the invention proposes a method of determining at least one radius of protection associated with a respective navigation parameter of a hybrid inertial navigation system by Kalman filtering including the steps of:                receiving at the filter input a safe position measurement and an inertial measurement from the hybrid inertial navigation system comprising a triaxial accelerometer measurement and a triaxial rate gyro measurement;        producing an inertial platform from the inertial measurements to obtain at least one navigation parameter from a set of parameters including the position, speed, attitude and altitude from the hybrid inertial navigation system;        effecting Kalman filtering including updating of a state vector including an estimate of the errors of the navigation parameter or parameters and a covariance matrix associated with those states, propagation of the state vector, hybridizing the inertial navigation data with the safe position received by means of the following iterative substeps:                    effecting a updating of the state vector based on an observation of the difference between the inertial measurement and the safe position measurement, wherein a position bias is introduced into the states model of the Kalman filter, representing the uncertainty associated with the input measured safe position;            decorrelating the state of said position bias of all of the other states by means of the following substeps:                            rendering the state of said position bias independent of the other states during two successive propagations by increasing the model noise of the position bias;                cancelling the correlations present in the matrix of the Kalman filter of this state with all the other states; and                reinitializing the covariance of the position bias state with the input safesafe position measurement uncertainty;                                    propagating the state vector until the covariance associated with the position error state estimated by the Kalman filter is equal to the covariance associated with the safe position bias;            computing the safe navigation parameter or parameters by adding to the inertial navigation parameters from the inertial platform the estimated navigation errors contained in the state vector of the Kalman filter; and            computing the radius or radii of protection associated with respective safe navigation parameter or parameters using the covariance matrix of the Kalman filter.                        
Such a method makes it possible to supply safe navigation parameters, i.e. parameters supplied with the associated radius of protection, without making hypotheses as to the measurements used apart from the maximum amplitude of the error that affects them.
In accordance with one embodiment, the step consisting in receiving at the input of the filter a safe position measurement uses a system combining inertia and a satellite navigation system.
In one embodiment, the step consisting in receiving at the input of the filter a safe position measurement uses a spatial augmentation system.
Another aspect of the invention proposes a system for determining at least one radius of protection associated with a respective navigation parameter of a hybrid inertial navigation system, including a Kalman filter adapted to execute the method as described above.
A further aspect of the invention proposes an aircraft including a system as described above.
In all the figures, elements having identical references are similar.